1. Field of the Invention
This invention relates to a highly water-absorptive powdery polymer composition.
The highly water-absorptive powdery polymer composition according to the present invention is improved in its reduced hygroscopicity and cohesiveness while maintaining various water absorption properties inherent in highly water-absorptive polymers. Particularly noteworthy is that the present powdery polymer composition is remarkably improved in its reduced adhesion to metals. Thus, it can advantageously be used for various water-absorptive materials.
2. Background Art
Highly water-absorptive polymers are synthetic polymers which have recently been used not only for sanitary goods or paper diapers in sanitary fields but also for water retentive materials, dew condensation preventive materials, freshness retentive materials and solvent dehydrating materials in industrial fields as well as in agricultural and horticultural fields, and are now expected to be applied in a wider range of fields. In general, such polymers are often processed into sheets especially when used for sanitary materials and in industrial applications. According to one process (i) applied to this end, a highly water-absorptive powdery polymer is inserted between two unwoven fabrics, which are then heat-sealed or otherwise processed into a water-absorptive sheet. According to another process (ii), cottony pulp is put on a sheet impermeable to liquid, a highly water-absorptive powdery polymer is scattered on that sheet, and cottony pulp and a sheet permeable to liquid are further placed on that sheet, followed by pressing into a water-absorptive sheet. According to a further process (iii), cottony pulp is previously mixed with a highly water-absorptive powdery polymer, and the resulting mixture is inserted between two sheets respectively permeable and impermeable to liquid, followed by pressing into a water-absorptive sheet. Other processes may also be used. In order to produce such water-absorptive sheets on an industrial scale, use is often made of machinery such as carriers, mixers, constant distributors and roll type pressing machines.
In this case, machine troubles are likely to occur due to the hygroscopicity, cohesiveness and adhesion to metals of highly water-absorptive powdery polymers. This is particularly marked when the polymers are those synthesized by reverse-phase suspension polymerization.
Various proposals have been made to decrease the hygroscopicity, cohesiveness and adhesion to metals of such highly water-absorptive powdery polymers. For instance, there have been proposed:
(a) granulation of powder into granules, PA1 (b) removal of fine powder by classification, PA1 (c) surface treatment for coating a hydrophobic substance on the surface of polymers, PA1 (d) mixing of polymers with specific hydrophobic ultrafine silica particles, as disclosed in Japanese Patent Kokai Publication No. 133028/1981.
So far as the present inventors know, however, the above method (a) needs complicated steps, and the method (b) is disadvantageous in the light of production yield. A problem with the method (c) is that the water absorption capacity of the polymer treated is excessively reduced. With the method (d), it is not easy to effect uniform mixing of polymers with such ultrafine particles. Taken altogether, the above methods (a) to (d) have some effects upon reduction in the hygroscopicity and cohesiveness of highly water-absorptive powdery polymers. However, they are little effective for reduction in the adhesion to metals of highly water-absorptive powdery polymers, especially with polymers prepared by reverse-phase suspension polymerization.
In other words, the hygroscopicity, cohesiveness and adhesion to metals of highly water-absorptive powdery polymers prepared by, esp., reverse-phase suspension polymerization are not simultaneously improved to satisfactory levels. In this regard, there is left much to be desired.